Single site catalysts such as metallocenes have received wide attention for their ability to make polyethylene and polypropylene having relatively narrow molecular weight distributions and uniform comonomer distributions at excellent polymerization rates. Recently, the narrow molecular weight distributions have been addressed and broadened out improving processability.
It is known that particular bridged metallocene catalysts containing cycloalkadienyl ligands epimerize into their racemic and meso forms in the presence of, for instance, light or amines with heating. The racemic form generates isotactic polypropylene, while the meso form produces atactic polypropylene. When bridged metallocene catalysts having cycloalkadienyl ligands are commercially used to make polypropylene, the meso stereoisomer is generally separated out, normally at great cost, to avoid the formation of atactic polypropylene to provide resin for applications in which atactic polypropylene is not desired.
U.S. Pat. No. 5,304,614 and Spaleck et al, "The Influence of Aromatic Substituents on the Polymerization Behavior of Bridged Zirconocene Catalysts", Organomet., 1994, Vol. 13, p. 954, disclose the use of bridged metallocene catalysts having cycloalkadienyl ligands in the production of polypropylene and polyethylene. Each of these references emphasizes the importance of separating out undesirable meso stereoisomers from the catalyst composition. Further, as described by Bercaw et al "Racemo-Meso Isomerization for Ansa-scandocene and Ansa-yttrocene Derivatives", 215th ACS Meeting, held in Dallas, Tex. between Mar. 29 and April 2, 1998, presentation # 059, the interconversion can occur more rapidly at higher temperatures due to more favorable energetics available such that systems which interconvert very slowly at room temperature, equilibrate rapidly above 55 degrees C. Since many of the industrial processes for polymerization are conducted at higher temperatures, there is a real possibility that interconversion can occur during polymerization, further producing the meso epimer and subsequently the atactic polymer this isomer is known to generate.
While there is an industrial need for a process utilizing a metallocene catalyst, which produces resins, which do not have an atactic component, there is a greater need for a process which gives the operator control over the atactic/isotactic components of polypropylene and polyethylene. This can be accomplished by selecting the cocatalyst, which selectively activates only the desired epimer or, alternatively, both epimers; thus, the resin properties (either polyethylene or polypropylene or other) can be varied when a mixture of epimers is present in the catalyst precursor by the correct selection of cocatalyst.